This invention pertains to the art of rock drilling with pressure fluid actuated percussion type drills wherein repeated impact blows are transmitted through a drill stem comprising one or more elongated rods or tubes coupled end to end and connected to a percussion bit which penetrates a rock formation by localized fracture and crushing of the rock structure.
It has been observed in pursuing the present invention that a rock formation of a particular hardness or compressive strength can be penetrated with the aforementioned type of drilling most efficiently, that is the greatest rate of hole formation for a given rate of energy input to the rock drill proper, at a particular impact blow energy value taking into consideration the configuration and size (diameter) of the percussion bit. An impact blow which has a low energy value will deflect the rock formation but not sufficiently to cause substantial fracture and breaking up of the rock structure. Accordingly, since most rock formations exhibit a stiffness characteristic and undergo elastic deflection when subjected to impacts a major portion of the energy of an impact blow imparted to the rock may be reflected back through the bit and the drill stem or dissipated into the rock formation without effecting very much rock fracture. Operation of a percussion drill at a hammer blow energy which is too low will result in very slow penetration or hole formation and an early failure of the drill stem components as well as substantial loss of the energy or power consumed in operating the drill.
Conversely, it is believed that if the impact blow energy is too high that penetration of the bit and breaking of the rock will occur but that at least some elastic compressive deflection of the drill stem and bit caused by the impacting of the hammer cannot be transmitted substantially to the rock formation once initial breaking and penetration has taken place because the bit will not remain in firm contact with the unbroken rock. Therefore, at least some of the impact blow energy cannot be transmitted to the rock formation and instead causes cyclical compression and elongation of the drill stem which is undesirable. If the impact blow energy is too high for a particular type of rock being drilled early fatigue failures of the drill stem components and bit is also experienced and energy is wasted.
It has been further observed that a rock formation of a particular compressive strength (as measured by uniaxial loading of a finite sample) requires a certain energy value to break out or remove a unit volume of rock by percussion drilling. It follows then that in percussion drilling of circular cross section holes with bits which have a fixed ratio of cutting edge length to bit diameter it would be desirable to maintain a fixed value of impact energy per unit of bit diameter for drilling holes of various sizes in a given type of rock. Accordingly, depending on hole size the total impact blow energy imparted to the bit by the hammer should be adjusted to provide the requisite blow energy for a given hole size which will be the most efficient or yield the greatest penetration rate for the power input to the drill proper.
In pursuing the present invention it has been determined that a percussion drill motor operated by hydraulic pressure fluid and capable of imparting to the drill stem and bit impact blows of variable intensity or energy value may be advantageous for drilling in different types of rock in the most efficient manner. Moreover, such a drill may also be used to drill more efficiently a range of hole sizes within the working limits of the drill system in regard to the impact blow energy delivered to the drill stem and bit and total power input to the drill which will not materially reduce the useful life of the drill or the drill stem components.
Percussion type rock drills are known which are capable of being controlled to deliver variable impact blow energy and blow frequency. Prior art drills are generally characterized by control devices which require direct access to the rock drill unit itself to effect a change in hammer stroke length and blow frequency. Prior art hammer stroke length and blow frequency controls are also generally characterized by devices which provide for a finite number of different drill operating frequencies and hammer stroke lengths none of which might be the most effective for drilling a particular type of rock in accordance with the foregoing observations.
Furthermore, in known drills of the type which operate on hydraulic pressure fluid supplied by a conventional motor driven pump the changes in fluid flow rate and supply pressure caused by changes in hammer stroke length or blow frequency do not permit operation of the drill unit at a substantially constant rate of hydraulic power input to the drill itself. Accordingly, the improvements in drill penetration rate for a particular type of rock or hole size which could be achieved with changing the impact blow energy are not realized because the necessary changes in fluid flow and pressure cannot be accomplished to provide a substantially constant hydraulic power input to the drill percussion mechanism.